This application claims the priority of German Patent Document 199 53 933.2 filed Nov. 10, 1999 and German Patent Document 299 23 272.7 filed Nov. 10, 1999, the disclosure of which are expressly incorporated by reference herein.
The invention relates to a method and an apparatus for adjusting an internal combustion engine with variable valve timing.
German Patent DE 195 30 274 A1, shows a piston-type internal combustion engine with fully variable valve gear, in which the opening and closing points of the engine valves can be controlled independently of one another. According to the method in DE 195 30 274 A1, the load input via the accelerator pedal is converted by means of a fuel control unit into a quantity of fuel to be injected, the injection point and ignition point being determined by means of corresponding maps. The required quantity of fresh air is controlled by controlling the engine valves. A lambda probe is used to measure the air ratio (lambda), which is used in an air control unit as the basis for control of the mass flow of fresh air. The air ratio is controlled by manipulating the opening times of the engine valves.
Because control of the air ratio is only possible to the maximum fresh air charge for the cylinder at certain operating points, the air control unit and the fuel control unit are linked in order to provide the possibility of reducing the quantity of fuel injected independently of the position of the accelerator pedal by multiplying the quantity of air determined by a correction factor. The correction factor is determined from a set/actual comparison of the quantity of incoming air. It is possible to determine the actual quantity of air by means of an air mass meter.
The disadvantage of this method is that, because of the measurement, and the control operation following the measurement the quantity of air to be supplied is adapted fundamentally with a delay, with the result that a fuel/air ratio is different from the stoichiometric ratio is set, especially in the region of dynamic operations, e.g. in the region of large changes in speed.
German Patent DE 195 30 274 A1 furthermore discloses converting the driver""s intention, to be input via the accelerator pedal, into a set charge in the cylinder as a function of the air temperature and the intake-pipe pressure. This set charge is being used to control the variable valve gear. Injection and ignition are controlled as a function of the quantity of air determined from the set charge.
Although this method takes into account the temperature and pressure of the induction air, DE 195 30 274 A1 indicates only a general dependence but does not give any specific instructions for calculating the required quantity of air. Moreover, there is no support for adaptation of the valve overlap and hence of the residual gas content or for switching to a different valve operating mode, with consideration of a suitable method, especially in the case of major compensating operations required by the environment.
The underlying problem faced by the inventors is to adjust the fuel/air ratio in an internal combustion engine rapidly and reliably in a wide operating range. In particular, the quantity of air is to be pilot-controlled as accurately as possible.
According to the present invention, the internal combustion engine is fitted both with variable valve timing for variable adjustment of engine valves and with a throttling device/valve in the intake pipe. To enable throttle-free adjustment and control of the internal combustion engine over wide ranges of operation, air is supplied to the combustion chambers of the internal combustion engine preferentially by manipulating the opening and/or closing points of the engine valves and for conventional throttle control to be resorted to only in ranges in which adjustment is not possible exclusively by means of the engine valves or in which thermodynamic advantages can be obtained.
As a criterion for deciding whether adjustment should be performed by means of the engine valves or the throttling device, the set pressure, taking into account the ambient temperature, is compared with a reference pressure, which is advantageously identical with the ambient pressure. If the set pressure exceeds the reference pressure, the set air mass is controlled by means of the engine valves with the throttling device completely open. If, on the other hand, the set pressure falls below the reference pressure, the set air mass is controlled by means of the throttling device.
As a function of the position of the accelerator pedal, a set air mass is advantageously first determined. This air mass is associated with a set pressure in the intake pipe, which is corrected by a correction factor proportional to the ambient temperature. If the temperature-corrected set pressure is lower than the reference pressurexe2x80x94in particular the current ambient pressurexe2x80x94the throttling device is activated. In this case, the ambient pressure upstream of the throttling device is reduced to the corrected set pressure by changing the position of the throttle valve, and the charge corresponding to the set air mass is supplied to the cylinders. This embodiment offers the advantage that it is possible to switch to conventional throttle-valve control, especially in ranges of low load and high engine speed, thereby making it possible to avoid vibration and inertia problems at this operating point with the valve timing system, which is preferably operated electromagnetically. Furthermore, thermodynamic advantages are also obtained. Outside the relatively small range in which throttling is advantageous or the intake-air temperature correction leads to anything up to complete opening of the throttle valve, throttle-free adjustment of the quantity of fresh air to be supplied is performed by means of the engine valves, preferably by adapting the inlet closing point of the inlet valve.
The closing point of the inlet valve is expediently corrected by means of a compensating function, the polynomial coefficients of which are read out from maps. The polynomial coefficients are ideally plotted against the engine speed in load or charge maps. The compensating function may have a linear or quadratic profile.
The compensating function is expediently provided with data in the form of corresponding polynomial coefficients for each valve operating mode. The switch to a new valve operating mode advantageously takes place after volumetric cylinder charging, thereby ensuring that the switch is always made at the same crank angle position, irrespective of the degree of compensation. All other control variables such as the opening point of the inlet valve, the opening and closing points of the exhaust valves, ignition and injection are furthermore coupled to the set air mass.
In an advantageous development, it may be expedient to limit the set air volume to be supplied via the engine valves, to which correction factors, that take account of the outside pressure and outside temperature, have been applied, to a predetermined maximum limits to prevent a larger corrected set air volume than would be possible at full load under standard conditions from being calculated at high engine loads. Capping the corrected set air volume to the permissible maximum limits the period of opening of the inlet valves, thereby avoiding displacement of the inlet closing point to an operating point that impairs combustion. It is then possible to work back reciprocally from the air volume in order to determine a cylinder-specific, maximum permissible limiting mass flow that can be used for ignition and injection.
Taking account of the differential volume, formed from the corrected air volume and the set air volume based on standard conditions, the inlet closing point of the inlet valve is expediently manipulated by means of a subsequent compensating function. This ensures that the corrected set air volume supplied to the cylinder corresponds to the set air mass under standard conditions. This has the advantage that all other map-dependent variables, such as ignition and injection and the opening and closing points of the remaining engine valves, can be retained.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.